The present invention relates generally to spray drying methods, and more particularly to one in which the effluent gases from the spray drying method are to be subjected to a particulate removal operation downstream of the spray dryer, as by scrubbing or electrostatic precipitation.
Generally, in conventional spray drying methods, a solids-containing, aqueous, liquid slurry is introduced into the top of a spray drying tower while a heated gas stream is introduced into the bottom of the tower. The descending slurry is dried by the rising, heated gas stream. Dried, solid particles from the slurry are removed in powdery form from the bottom of the spray drying tower, and effluent gases are removed from the top of the spray drying tower.
A typical, conventional spray drying method is disclosed in Hall U.S. Pat. No. 1,985,987 entitled "Art of Spray Drying Soap". As noted in Hall (Column 2, lines 30-38), the effluent gases removed from the top of a spray drying tower includes, as contaminants, particles of the very same powder as is removed from the bottom of the spray drying tower. Because of the presence of these and other contaminants in the effluent gases from the spray drying tower, the effluent gases cannot be discharged into the atmosphere without impermissible pollution of the atmosphere, so that the contaminated effluent gases must be treated downstream of the spray drying tower to remove the contaminants, e.g., by scrubbing or electrostatic precipitation, or both. These are common processes for removing solid particulates and fumes from gases.
It is well known in the art of removing contaminants from gases that the contaminants in the gas vary with the particular industrial process from which the gas originates. Similarly, the scrubbing liquid utilized depends upon the particular contaminants in the gas. To remove solid particulates, water is the conventional scrubbing liquid. When the gases contain fumes which are acidic, an alkaline solution is conventionally utilized as part of the scrubbing liquid. Similarly, when the gases are contaminated with alkaline fumes, an acidic solution is utilized in the scrubbing liquid. This is evidenced by the following prior art patents:
Hausberg U.S. Pat. No. 3,140,163 (Water is the usual scrubbing liquid for removing solid particles from gas--Column 1, lines 14-17);
Moser U.S. Pat. No. 1,128,177 (Water is used to remove solid impurities from smoke--Page 1, column 1, lines 8-21, page 2, column 2, lines 67-83);
Mare U.S. Pat. No. 3,626,667 (Water is conventionally used as scrubbing liquor in a scrubber to remove solid particles from gas--Column 1, lines 34-51); and
Heitz U.S. Pat. No. 2,747,962 (Water or alkaline solution are customarily used to remove acidic gases from gas mixtures--Column 1, lines 15-25).